Process for producing golf ball

ABSTRACT

Golf ball  1  includes a core  2  and a cover  3.  The core  2  has two layers including a center  4  and a mid layer  5.  Upon molding the core  2,  the center  4  covered by two half shells are first placed into a mold. Next, the lower portion of the mold moves upwards at a moving velocity of equal to or greater than 3.0 mm/s. The upward movement is terminated at a time point when the space between the upper portion and the lower portion becomes from 5 mm to 30 mm. Next, the lower portion is further moved upwards at a moving velocity of from 0.03 mm/s to 1.0 mm/s while having the rubber composition outflow form the cavity. The upper portion and the lower portion are then brought into contact. Next, the rubber composition within the cavity is crosslinked to form the mid layer  5.  At the same time, the rubber composition that flowed out is cured to form a ring-shaped molding flash. The mold is unclamped, and the core  2  and the molding flash are removed.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to processes for producing a golfball. More particularly, the present invention relates improvement ofcompression molding processes of a golf ball or a spherical body thatforms a part of a golf ball.

[0003] 2. Description of the Related Art

[0004] There exist solid golf balls having a variety of layeredstructure. For example, there are one-piece golf balls including asingle layer, two-pieces golf balls including a cover and a core havinga single-layered structure, and multi-pieces golf balls including acover and a core having a two-layered structure. The core having atwo-layered structure including a center and a mid layer covering overthis center. Any of the one-piece golf balls, the cores for a two-piecesgolf ball, and the centers and the cores for a multi-pieces golf ballare spherical bodies, which are usually formed from a rubbercomposition.

[0005] When a core having a two-layered structure is formed, a mold isprovided comprising upper and lower portion both of which having anumber of hemispheric cavities. Then, a center and a rubber compositiondisposed surrounding this center are placed into a cavity (in usual, thecavity of the lower portion). Total volume of the rubber composition andthe center is greater than the volume of the core. As the mold isgradually clamped, excess rubber composition outflows from the cavities.Concomitant with the outflow of the rubber composition, air is alsodischarged from the cavities. The mold is completely clamped and heatedto a predetermined temperature, and then kept for a predeterminedperiod. Crosslinking reaction of the rubber is thereby caused to curethe rubber composition, and thus a mid layer is formed. Thereafter, themold is unclamped, and the core is removed from the cavity. The rubbercomposition flowed out from the cavity is also cured to some extent,which resides around the cavity in a ring-shape. The cured matter isreferred to as a “molding flash”. The molding flash is removed from themold.

[0006] Because the mold has a number of cavities as described above, anumber of molding flashes in a ring-shape are also formed. Number of themolding flashes which are formed are identical with the number of thecavities. Usually, from 4 to 100 molding flashes are formed per onemold. Adjacent molding flashes contact on each other, and integrated.Therefore, all of the molding flashes are serially connected to resultin the formation of a sheet-like shape. An operator grasps and pulls apart of the molding flashes, thereby the entire flash can be readilyremoved at once.

[0007] Upon clamping, if a large amount of the rubber compositionoutflows in an omnidirectional manner, adjacent molding flashes can beintegrated on each other without fail. However, the outflow of a largeamount of rubber composition leads directly to decreased process yield,thereby elevating costs for producing a golf ball. To the contrary, ininstances where the amount of the outflow is suppressed, some partsamong the adjacent molding flashes are not integrated, if the amount ofthe outflow varies in dependence upon the direction. Accordingly,working property is deteriorated in removing the molding flash. Inaddition, varying amount of the outflow also leads to deviation of massof the mid layer (uneven wall thickness).

[0008] Problems involving in the deteriorated working property inremoving the molding flash are also raised upon the compression moldingof one-piece golf balls and the cores for a two-pieces golf ball, thecenters for a multi-pieces golf ball, and the like.

SUMMARY OF THE INVENTION

[0009] A process for producing a golf ball according to the presentinvention comprises the following steps (A) to (C):

[0010] (A) a placing step wherein a rubber composition is placed into acavity of a mold comprising upper and lower portion both of which havinga number of hemispheric cavities, in a state where the mold isunclamped;

[0011] (B) a clamping step wherein the upper portion and the lowerportion are brought into contact by having the lower portion relativelyclose to the upper portion at a moving velocity of from 0.03 mm/s to 1.0mm/s while having the rubber composition outflow form the cavities; and

[0012] (C) a removing step wherein the mold is unclamped, and sphericalbodies formed in the cavities and the rubber composition are removed.

[0013] In accordance with this process for producing a golf ball, upperportion and lower portion get close at an extremely low velocity in theclamping step. When the velocity in getting close is rapid, the rubbercomposition intensively outflows to a direction that is liable tooutflow, however, when the velocity is set to be so slow, the rubbercomposition uniformly outflows to any direction around the cavity.Accordingly, adjacent molding flashes surely join on each other eventhough the amount of outflow is suppressed, and thus resulting infacilitated operation for removing the molding flashes.

[0014] Preferably, the process for producing a golf ball comprises thefollowing step (D) between the placing step (A) and the clamping step(B) described above:

[0015] (D) an approaching step to have the lower portion relativelyclose to the upper portion at a moving velocity of equal to or greaterthan 3.0 mm/s.

[0016] In this process for producing a golf ball, the moving velocity inthe approaching step (D) is relatively rapid. Therefore, prolongedmolding cycle time can be suppressed in spite of the presence of theclaming step (B).

[0017] Preferably, the clamping step (B) is initiated at a time pointwhen the space between the upper portion and the lower portion becomesfrom 5 mm to 30 mm. In accordance with this process for producing a golfball, uniform outflow of the rubber composition and shortened cycle timecan be both achieved to more extent.

[0018] Preferably, percentage ratio of volume of the matter placed intothe spherical cavity to volumetric capacity of the spherical cavityformed from cavity of the upper portion and cavity of the lower portionis from 103% to 120%.

[0019] The process for producing a golf ball according to the presentinvention exerts especially remarkable effects when the rubbercomposition is placed into the cavity with the spherical center in astate where the rubber composition is positioned surrounding thiscenter.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 is a schematic view illustrating a golf ball obtained by aprocess for production according to one embodiment of the presentinvention with a partially cut off cross-section;

[0021]FIG. 2 is a cross sectional view illustrating a part of a mold foruse in the production of the golf ball shown in FIG. 1;

[0022]FIG. 3 is a flowchart illustrating one example of a process forproducing the golf ball shown in FIG. 1;

[0023]FIG. 4 is a cross sectional view illustrating a part of a mold ata stage when the clamping step is completed in the process forproduction shown in FIG. 3; and

[0024]FIG. 5 is a perspective view illustrating a number of moldingflashes formed in accordance with the process for production shown inFIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] The present invention is hereinafter described in detail withappropriate references to the accompanying drawing, according to thepreferred embodiments of the present invention.

[0026] A golf ball 1 depicted in FIG. 1 has a core 2 and a cover 3. Thecore 2 has two layers including a center 4 and a mid layer 5. Numerousdimples 6 are formed on the surface of the cover 3. This golf ball 1 hasa paint layer and a mark layer on the outer surface of the cover 3,although not shown in the Figure. This golf ball 1 has a diameter offrom 40 mm to 45 mm, and in particular, of from 42 mm to 44 mm. In lightof the reduction of air resistance in the range to comply with a ruledefined by United States Golf Association (USGA), the diameter ispreferably from 42.67 mm to 42.80 mm. Further, this golf ball 1 has aweight of from 44 g to 46 g, and in particular, of from 45.00 g to 45.93g.

[0027]FIG. 2 is a cross sectional view illustrating a part of a mold 7for use in the production of the golf ball 1 shown in FIG. 1. The mold 7comprises an upper portion 8 and a lower portion 9. Each of the upperportion 8 and lower portion 9 has a number of cavity faces 10. Asemispherical cavity is formed by this cavity face 10. When the upperportion 8 and the lower portion 9 are brought into contact, sphericalcavities are formed.

[0028]FIG. 3 is a flowchart illustrating one example of a process forproducing the golf ball shown in FIG. 1. In this process for production,a base rubber, a crosslinking agent, additives and the like are firstkneaded (STP 1) to give a rubber composition. Next, this rubbercomposition is placed into a spherical cavity of a mold followed bycompression and heating. Accordingly, a spherical center 4 is formed(STP2). In general, heating initiates across linking reaction, therebythe rubber composition is cured. As a matter of course, the center 4maybe either uncrosslinked or semicrosslinked in this stage.

[0029] Next, a base rubber, a crosslinking agent, additives and the likeare kneaded (STP3) to give another rubber composition. Thereafter, halfshells 11 shown in FIG. 2 are formed from this rubber composition(STP4). In order to form the half shell 11, a mold comprising ahemispherical cavity and a hemispherical protrusion is usually utilized.

[0030] Next, the center 4 are covered with two half shells 11, and thesehalf shells 11 and the center 4 are placed into a mold 7 as shown by anarrow head in FIG. 2 (STP5). This step (STP5) is referred to as aplacing step. Half shells 11 and the center 4 are usually placed intothe cavity of the lower portion 9.

[0031] Next, the lower portion 9 gradually moves upwards to get close tothe upper portion 8 (STP6). This step (STP6) is referred to as anapproaching step. Relative moving velocity of the lower portion 9 towardthe upper portion 8 in the approaching step (STP6) is comparativelyrapid. Cycle time can be thereby shortened.

[0032] Next, upward movement of the lower portion 9 is terminated (STP7)Then, the lower portion 9 moves upwards again (STP8). This step (STP8)is referred to as a clamping step. Relative moving velocity of the lowerportion 9 toward the upper portion 8 in the approaching step (STP8) isrelatively slow. In the clamping step (STP8), the upper portion 8 andthe lower portion 9 gradually get close. Compression of the rubbercomposition (half shell 11) is accompanied thereby. Generally,temperature of the mold 7 is elevated concurrently with the compressionto heat the rubber composition. In accordance with the compression andheating, the rubber composition in the cavity flows, and thus excessrubber composition gradually outflows from the cavity. At the stage whenthe clamping step (STP8) is completed, the upper portion 8 and the lowerportion 9 are brought into contact. Thereafter, the rubber compositionhardly outflows.

[0033] After the clamping step (STP8) is completed, the mold 7 is keptat a predetermined temperature for a predetermined time period.Accordingly, crosslinking reaction of the rubber proceeds (STP9), whichresults in curing of the rubber composition to form the mid layer 5.When the center 4 is uncrosslinked or semicrosslinked, the center 4 isconcurrently crosslinked in this step (STP9). In general, temperature ofthe mold 7 in common crosslinking step (STP9) is set to be from 140° C.to 180° C. Time period for the crosslinking is set to be from 10 minutesto 40 minutes.

[0034] Next, the mold 7 is unclamped, and the core 2 having the center 4and the mid layer 5 is removed from the cavity. At the same time, therubber composition flowed out from the cavity is also removed. This stepis referred to as a removing step (STP10). The cover 3 is formed aroundthe removed core 2 by a known procedure (e.g., an injection moldingmethod) (STP11), to obtain the golf ball 1.

[0035]FIG. 4 is a cross sectional view illustrating a part of a mold 7at a stage when the clamping step (STP8) is completed in the process forproduction shown in FIG. 3. As is clear from this Figure, the rubbercomposition that flowed out from the cavity adheres onto an upper faceof the lower portion 9. The rubber composition that flowed out from eachof the cavities is ring-shaped, which is cured to some extent in thecrosslinking step (STP9). This cured matter is referred to as a moldingflash 12. As is shown in FIG. 4, a molding flash 12 formed around agiven cavity and a molding flash 12 formed around the adjacent cavityare contacted on each other, and are integrated.

[0036]FIG. 5 is a perspective view illustrating a number of moldingflashes 12 formed in accordance with the process for production shown inFIG. 3. As described above, each of the molding flashes is ring-shaped.Adjacent molding flashes 12 join on each other, which exhibit a form ofa single sheet. All of the molding flashes 12 are removed from the mold7 at once by pulling up one molding flash 12 which is present on anupper face of the lower portion 9 of the mold. This removing operationis markedly simple.

[0037] As described above, relative moving velocity of the lower portion9 toward the upper portion 8 in the clamping step (STP8) is markedlyslow. Accordingly, the rubber composition outflows in a nearly uniformmanner from the periphery of the cavity, and thus the integration amongadjacent molding flashes 12 can be attempted. In addition, deviation ofmass of the mid layer 5 can be suppressed. Moving velocity maybe set tobe from 0.03 mm/s to 1.0 mm/s. When the moving velocity is below therange described above, cycle time in the process for producing the golfball is prolonged, resulting in insufficient productivity. In thisrespect, the moving velocity is more preferably equal to or greater than0.05 mm/s, and particularly preferably equal to or greater than 0.1mm/s. When the moving velocity is beyond the range described above,outflow of the rubber composition may not be uniform, thereby causingpositions where joining among the adjacent molding flashes 12 cannot beachieved. In this respect, the moving velocity is more preferably equalto or less than 0.8 mm/s, and particularly preferably equal to or lessthan 0.6 mm/s. In order to relatively approach the lower portion 9toward the upper portion 8, the lower portion 9 maybe moved upwards, orthe upper portion 8 maybe moved downwards. Alternatively, both of theupward movement of the lower portion 9 and the downward movement of theupper portion 8 maybe simultaneously executed. In any instance, relativemoving velocity is set to be in the range described above. The movementcan be usually achieved by a pressing machine.

[0038] In the approaching step (STP6), pressure against the half shells11 is scarcely imparted, and thus flow of the rubber composition hardlyoccurs. Therefore, even though the moving velocity in the approachingstep (STP6) is set to be so rapid, no adverse effects are exertedagainst uniform outflow of the rubber composition. Relative movingvelocity of the lower portion 9 toward the upper portion 8 in theapproaching step (STP6) is preferably equal to or greater than 3.0 mm/s,and particularly preferably equal to or greater than 8.0 mm/s. Cycletime can be thereby shortened. In general, the moving velocity is set tobe equal to or less than 120 mm/s.

[0039] It is preferred that the clamping step (STP8) is initiated at atime point when the space between the upper portion 8 and the lowerportion 9 becomes from 5 mm to 30 mm. When this step is initiated at aposition with a less space of the range described above, half shells 11are compressed to result in the initiation of outflow of the rubbercomposition at a stage of the approaching step (STP6), in other words,at a stage when the moving velocity is still rapid. Accordingly, theoutflow may lack uniformity. In this respect, the space upon initiationis more preferably equal to or greater than 7 mm, and particularlypreferably equal to or greater than 10 mm. When this step is initiatedat a position with a greater space of the range described above, cycletime maybe prolonged, resulting in deteriorated productivity of the golfball 1. In this respect, the space upon initiation is more preferablyequal to or less than 25 mm, and particularly preferably equal to orless than 20 mm.

[0040] In the process for production shown in FIG. 3, the movement ofthe lower portion 9 is stopped during the time period between theapproaching step (STP6) and clamping step (STP8), however, the clampingstep (STP8) maybe conducted successively to the approaching step (STP6).The clamping step (STP8) maybe initiated in a state where the mold 7 isunclamped utmost, without providing the approaching step (STP6).

[0041] It is preferred that percentage ratio of volume of the matterplaced into the spherical cavity (in this case, summation of volume ofthe center 4 and volume of two half shells 11) to volumetric capacity ofthe spherical cavity is from 103% to 120%. When the percentage ratio isless than the above range, there arise difficulties in integration amongthe adjacent molding flashes 12. In this respect, the percentage ratiois particularly preferably equal to or greater than 105%. When thepercentage ratio is greater than the above range, costs for thematerials of the golf ball 1 are elevated. In this respect, thepercentage ratio is particularly preferably equal to or less than 110%.Inside diameter of the cavity usually ranges from 15 mm to 45 mm, andparticularly, from 38 mm to 43 mm.

[0042] As a base rubber of the center 4, polybutadienes, polyisoprenes,styrene-butadiene copolymers, ethylene-propylene-diene copolymers(EPDM), natural rubbers and the like are suitable. Two or more kinds ofthese rubbers maybe used in combination. In view of the resilienceperformance of the golf ball 1, polybutadienes are preferred. Even inthe case where another rubber is used in combination with apolybutadiene, to employ a polybutadiene as a predominant component ispreferred. Specifically, it is preferred that the ratio of thepolybutadiene in total base rubber is equal to or greater than 50% byweight, and in particular, equal to or greater than 80% by weight. Amongpolybutadienes, high cis-polybutadienes are preferred, which have apercentage of cis-1, 4 bond of equal to or greater than 40%, and inparticular, equal to or greater than 80%.

[0043] Although mode of crosslinkage in the center 4 is not particularlylimited, it is preferred that a monovalent or divalent metal salt ofα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms is used as aco-crosslinking agent in view of the resilience performance. Specificexamples of the preferable co-crosslinking agent include zinc acrylate,magnesium acrylate, zinc methacrylate and magnesium methacrylate. Inparticular, zinc acrylate is preferred which can result in highresilience performance. An α,β-unsaturated carboxylic acid having 3 to 8carbon atoms and a metal oxide may also be used.

[0044] The amount of the co-crosslinking agent to be blended ispreferably from 10 parts to 40 parts per 100 parts of the base rubber.When the amount to be blended is below the above range, the center 4maybe so soft that insufficient resilience performance maybe achieved.In this respect, the amount to be blended is more preferably equal to orgreater than 15 parts, and particularly preferably equal to or greaterthan 20 parts. When the amount to be blended is beyond the above range,the center 4 maybe so hard that soft feel at impact may not beexperienced. In this respect, the amount to be blended is morepreferably equal to or less than 35 parts, and particularly preferablyequal to or less than 30 parts. The term “parts” used herein refers toweight ratio, i.e., parts by weight.

[0045] In the rubber composition for use in the center 4, an organicperoxide maybe preferably blended. The organic peroxide serves as acrosslinking agent in conjunction with the above-mentioned metal salt ofα,β-unsaturated carboxylic acid, and also serves as a curing agent. Byblending the organic peroxide, resilience performance of the center 4maybe improved. Suitable organic peroxide includes dicumyl peroxide,1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane,2,5-dimethyl-2,5-di(t-butylperoxy)hexane and di-t-butyl peroxide.Particularly versatile organic peroxide is dicumyl peroxide.

[0046] The amount of the organic peroxide to be blended is preferablyfrom 0.1 parts to 3.0 parts per 100 parts of the base rubber. When theamount to be blended is below the above range, the center 4 may be sosoft that insufficient resilience performance maybe achieved. In thisrespect, the amount to be blended is more preferably equal to or greaterthan 0.2 parts, and particularly preferably equal to or greater than 0.5parts. When the amount to be blended is beyond the above range, thecenter 4 maybe so hard that soft feel at impact may not be experienced.In this respect, the amount to be blended is more preferably equal to orless than 2.8 parts, and particularly preferably equal to or less than2.5 parts.

[0047] The rubber composition may also be blended with a filler foradjusting specific gravity thereof. Examples of the suitable fillerinclude inorganic salts such as zinc oxide, barium sulfate, calciumcarbonate and the like; and powder of highly dense metal such astungsten, molybdenum and the like. The amount of the filler to beblended is determined ad libitum so that the intended specific gravityof the center 4 can be accomplished. Preferable filler is zinc oxidebecause it serves not only as an agent for adjusting specific gravitybut also as a crosslinking activator.

[0048] Various additives such as anti-aging agents, coloring agents,plasticizers, dispersant and the like maybe blended at an appropriateamount to the rubber composition as needed.

[0049] Specific gravity of the center 4 usually ranges from 1.05 to1.25. Diameter of the center 4 usually ranges from 15 mm to 38 mm. Thecenter 4 may have two or more layers.

[0050] For the mid layer 5, a rubber composition which is similar tothat for the center 4 maybe used. Thickness T of the mid layer ispreferably from 0.5 mm to 10 mm. The mid layer 5 having a thickness T ofless than the above range involves difficulties in molding. In thisrespect, the thickness T is particularly preferably equal to or greaterthan 0.6 mm. When the thickness T is beyond the above range,insufficient resilience performance of the golf ball 1 maybe achieved.In this respect, thickness T is more preferably equal to or less than9.0 mm, particularly preferably equal to or less than 6.0 mm.

[0051] The cover 3 usually contains a synthetic resin as a predominantcomponent. As preferable synthetic resins in view of a feel at impactand resilience performance, ionomer resins and thermoplastic elastomers,and mixtures thereof are exemplified.

[0052] Of the ionomer resins, copolymers of α-olefin and α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms in which partof the carboxylic acid is neutralized with a metal ion are suitable. Asthe α-olefin herein, ethylene and propylene are preferred. Acrylic acidand methacrylic acid are preferred as the α, β-unsaturated carboxylicacid. Metal ions for the neutralization include: alkaline metal ionssuch as sodium ion, potassium ion, lithium ion and the like; bivalentmetal ions such as zinc ion, calcium ion, magnesium ion and the like;trivalent metal ions such as aluminum ion, neodymium ion and the like.The neutralization may also be carried out with two or more kinds ofmetal ions. In light of the resilience performance and durability of thegolf ball 1, particularly suitable metal ion is sodium ion, zinc ion,lithium ion and magnesium ion.

[0053] Specific examples of suitable ionomer resin include “Himilan1555”, “Himilan 1557”, “Himilan 1601”, “Himilan 1605”, “Himilan 1652”,“Himilan 1705”, “Himilan 1706”, “Himilan 1707”, “Himilan 1855”, “Himilan1856”, trade names by Mitsui-Dupont Polychemical Co. Ltd.; “Surlyn®9945”, “Surlyn® 8945”, “Surlyn® AD8511”, “Surlyn® AD8512”, trade namesby Dupont; and “IOTEK 7010”, “IOTEK 8000”, trade names by ExxonCorporation, and the like. Two or more ionomer resins maybe used incombination.

[0054] Preferable thermoplastic elastomers include thermoplasticpolyurethane elastomers, thermoplastic polyamide elastomers,thermoplastic polyester elastomers, thermoplastic styrene elastomers,thermoplastic elastomers having OH groups at their ends, and the like.Two or more thermoplastic elastomers maybe used in combination. In lightof the resilience performance of the golf ball 1, thermoplasticpolyester elastomers and thermoplastic styrene elastomers areparticularly suitable.

[0055] Thermoplastic styrene elastomers (thermoplastic elastomerscontaining styrene blocks) include styrene-butadiene-styrene blockcopolymers (SBS), styrene-isoprene-styrene block copolymers (SIS),styrene-isoprene-butadiene-styrene block copolymers (SIBS), hydrogenatedSBS, hydrogenated SIS and hydrogenated SIBS. Exemplary hydrogenated SBSinclude styrene-ethylene-butylene-styrene block copolymers (SEBS).Exemplary hydrogenated SIS include styrene-ethylene-propylene-styreneblock copolymers (SEPS). Exemplary hydrogenated SIBS includestyrene-ethylene-ethylene-propylene-styrene block copolymers (SEEPS).

[0056] Illustrative examples of thermoplastic polyurethane elastomersinclude “Elastolan”, trade name by BASF Polyurethane Elastomers Co.,Ltd., and more specifically, “Elastolan ET880” can be exemplified.Illustrative examples of thermoplastic polyamide elastomers include“Pebax®”, trade name by Toray Industries, Inc., and more specifically,“Pebax® 2533” can be exemplified. Illustrative examples of thermoplasticpolyester elastomers include “Hytrel®”, trade name by Dupont-Toray Co.,Ltd., and more specifically, “Hytrel® 3548” and “Hytrel® 4047” can beexemplified. Illustrative examples of thermoplastic styrene elastomersinclude “Rabalon®”, trade name by Mitsubishi Chemical Corporation, andmore specifically, “Rabalon® SR04” can be exemplified.

[0057] To the cover 3, a diene block copolymer maybe blended. A dieneblock copolymer has double bonds derived from a conjugated dienecompound of a block copolymer or a partially hydrogenated blockcopolymer. The block copolymer comprises a polymer block of which basisbeing at least one vinyl aromatic compound, and a polymer block of whichbasis being at least one conjugated diene compound. The partiallyhydrogenated block copolymer can be obtained by hydrogenation of theblock copolymer as described above.

[0058] Exemplary vinyl aromatic compounds that constitute the blockcopolymer include styrene, α-methylstyrene, vinyltoluene,p-t-butylstyrene and 1,1-diphenylstyrene, and one or more compounds areselected from these. Particularly, styrene is suitable. Conjugated dienecompounds include butadiene, isoprene, 1,3-pentadiene and2,3-dimethyl-1,3-butadiene, and one or more compounds are selected fromthese. In particular, butadiene and isoprene, and a combination thereofare suitable.

[0059] Preferable diene block copolymer includes: those of whichstructure being SBS (styrene-butadiene-styrene) having polybutadieneblocks containing epoxy groups, and those of which structure being SIS(styrene-isoprene-styrene) having polyisoprene blocks containing epoxygroups. Illustrative examples of diene block copolymer include“Epofriend®”, trade name by Daicel Chemical Industries, Ltd., and morespecifically, “Epofriend® A1010” can be exemplified.

[0060] Coloring agents such as titanium dioxide and the like, fillerssuch as barium sulfate and the like, dispersants, anti-aging agents,ultraviolet absorbers, light stabilizers, fluorescent agents,fluorescent bleaching agents and the like maybe blended at anappropriate amount in the cover 3 as needed. Specific gravity of thecover 3 usually ranges from 0.9 to 1.4.

[0061] Thickness of the cover 3 preferably ranges from 0.5 mm to 2.5 mm.The cover 3 having thickness of below the above range is involved indifficulties in molding, and moreover, deteriorated durability of thegolf ball 1 maybe accompanied thereby. In this respect, the thickness ismore preferably equal to or greater than 1.0 mm, and particularlypreferably equal to or greater than 1.1 mm. When the thickness is beyondthe above range, unpleasant feel at impact maybe experienced. In thisrespect, the thickness is particularly preferably equal to or less than2.4 mm. The cover 3 may have two or more layers. Another mid layer maybeformed between the cover 3 and the mid layer 5.

[0062] The process for production according to the present invention, inwhich the moving velocity in the clamping step is set to be slow, issuitable for molding one-piece golf balls, cores for a two-pieces golfball, as well as centers for a three-pieces golf ball, and the like.When a one-piece golf ball is formed, a mold is used with the insidediameter of the cavity being from 40 mm to 45 mm. When a core of atwo-pieces golf ball is formed, a mold is used with the inside diameterof the cavity being from 38 mm to 43 mm. When a center of a three-piecesgolf ball is formed, a mold is used with the inside diameter of thecavity being from 15 mm to 40 mm. In any case, crosslinking temperatureis set to be from 120° C. to 180° C., whilst the crosslinking timeperiod set to be from 10 minutes to 40 minutes.

EXAMPLES Example 1

[0063] A rubber composition was obtained by kneading 100 parts ofpolybutadiene (“BR01”, trade name by JSR Corporation), 25 parts of zincacrylate, 22 parts of zinc oxide and 1.0 part of dicumyl peroxide in aninternal kneading machine. This rubber composition was placed in a moldhaving a spherical cavity, kept at 142° C. for 23 minutes and furtherkept at 168° C. for 6 minutes to obtain a center having a diameter of30.2 mm.

[0064] Next, a rubber composition was obtained by kneading 80 parts ofpolybutadiene (“BR01”, trade name, supra), 20 parts of anotherpolybutadiene (“BR10”, trade name, supra), 34 parts of zinc acrylate,16.3 parts of zinc oxide and 1.0 part of dicumyl peroxide in an internalkneading machine. This rubber composition was placed into a mold andcompressed to give a half shell. The center was covered with two of thehalf shells, and the center and the half shells were placed into a moldshown in FIG. 2. Thereafter, the lower portion was moved upwards at amoving velocity of 10 mm/s until the space between the upper portion andthe lower portion becomes 15mm, and then the movement was terminated.Immediately after this operation, the lower portion was further movedupwards at a moving velocity of 0.5 mm/s while having the rubbercomposition outflow from the cavities, and the upper portion and thelower portion were brought into contact. Then, crosslinking reaction ofthe rubber was caused to form a core including a center and a mid layer.Employed crosslinking temperature was 150° C., whilst the crosslinkingtime period was 20 minutes. Thereafter, the mold was unclamped, and acore and a molding flash were removed.

Examples 2 to 5 and Comparative Example 1

[0065] In a similar manner to Example 1 except that the moving velocityin the clamping step was altered as represented in Table 1 below, a corewas formed.

Examples 6 to 10

[0066] In a similar manner to Example 1 except that the space betweenthe upper and lower portions upon initiation of the clamping step wasaltered as represented in Table 2 below, a core was formed.

Comparative Example 2

[0067] In a similar manner to Example 1 except that the upper portionand the lower portion were brought into contact by moving the lowerportion upwards at a moving velocity of 10 mm/s, from a state where themold is unclamped utmost, a core was formed.

Example 11

[0068] In a similar manner to Example 1 except that the upper portionand the lower portion were brought into contact by moving the lowerportion upwards at a moving velocity of 1.0 mm/s, from a state where themold is unclamped utmost, a core was formed.

Evaluation of Working Property

[0069] Facility in removing operation of the molding flash was evaluatedby an operator. A case where the molding flash was extremely ready to beremoved, it was evaluated as “A”; a case where the molding flash wasrather difficult to be removed, it was evaluated as ‘B’; and a casewhere the molding flash was difficult to be removed and thus long timeperiod is required for the operation was evaluated as “C”. The resultsof evaluation are shown in Table 1 and Table 2 below.

Evaluation of Mass Deviation

[0070] Four positions for the measuring point equally spaced wereselected along a seam on the surface of the mid layer, and further,additional positions for the measuring point were also selected whichcorresponded to bipolar points when this seam is assumed as an equatorof a globe. Accordingly, thickness of the mid layer was measured at 6positions in total. A maximum value among 6 data was determined as Tmax,while the minimum value among them was determined as Tmin. Deviation ofmass was determined as a value: (Tmax-Tmin). Average values of deviationof mass from 10 golf balls was calculated, and the case where thisaverage value was less than 0.5 mm was evaluated as “A”, and the casewhere this average value was equal to or greater than 0.5 mm wasevaluated as “B”. The results of evaluation are shown in Table 1 andTable 2 below. TABLE 1 Results of Evaluation Example Example ExampleExample Example Comparative 2 3 1 4 5 Example 1 Moving velocity inapproaching step (mm/s) 10 10 10 10 10 10 Space upon initiation ofclamping step (mm) 15 15 15 15 15 15 Moving velocity in clamping step(mm/s) 0.03 0.05 0.5 0.8 1.0 2.0 Working property in removing moldingflash A A A A B C Cycle time slightly long slightly long short shortshort short Deviation of mass A A A A A B

[0071] TABLE 2 Results of Evaluation Example Example Example ExampleExample Comparative Example 6 7 8 9 10 Example 2 11 Moving velocity inapproaching step (mm/s) 10 10 10 10 10 10 1.0 Space upon initiation ofclamping step (mm) 3 5 20 30 40 — — Moving velocity in clamping step(mm/s) 0.5 0.5 0.5 0.5 0.5 10 1.0 Working property in removing moldingflash B A A A A C B Cycle time short short short short slightly longshort long Deviation of mass A A A A A B A

[0072] As is shown in Table 1 and Table 2, in accordance with theprocess for production of the Examples, working property was excellent,and deviation of mass of resulting mid layer is small. Accordingly,advantages of the present invention are clearly indicated by theseresults of evaluation.

[0073] The description herein above is merely for illustrative examples,and therefore, various modifications can be made without departing fromthe principles of the present invention.

What is claimed is:
 1. A process for producing a golf ball comprising: aplacing step wherein a rubber composition is placed into a cavity of amold comprising upper and lower portion both of which having a number ofhemispheric cavities, in a state where the mold is unclamped; a clampingstep wherein the upper portion and the lower portion are brought intocontact by having the lower portion relatively close to the upperportion at a moving velocity of from 0.03 mm/s to 1.0 mm/s while havingthe rubber composition outflow form the cavities; and a removing stepwherein the mold is unclamped, and spherical bodies formed in thecavities and the rubber composition are removed.
 2. The process forproducing a golf ball according to claim 1 further comprising betweensaid placing step and said clamping step: an approaching step to havethe lower portion relatively close to the upper portion at a movingvelocity of equal to or greater than 3.0 mm/s.
 3. The process forproducing a golf ball according to claim 2, wherein said clamping stepis initiated at a time point when the space between the upper portionand the lower portion becomes from 5 mm to 30 mm.
 4. The process forproducing a golf ball according to claim 1, wherein percentage ratio ofvolume of the matter placed into the spherical cavity to volumetriccapacity of the spherical cavity formed from said cavity of the upperportion and said cavity of the lower portion is from 103% to 120%. 5.The process for producing a golf ball according to claim 1, wherein therubber composition is placed into the cavity with a spherical center ina state where the rubber composition is positioned surrounding saidcenter in said placing step.